1. Field of the Invention
This invention relates to a differential gear device for a vehicle and more particularly to a differential gear device with a limited slip differential device.
2. Description of the Prior Art
Conventionally, in the differential gear device used for vehicles such as automobiles, it has been well known that many kind of limited slip differential devices (LSD) have been used in order to limit the differential movement in differential gearing.
These differential gear device include a device shown in U.S. Pat. No. 4,781,078. This differential gear device uses an electromagnet as an actuator in order to control the limiting force to limit the differential movement in the differential gearing by an operation at an outside. This differential gear device includes an electromagnet secured to an outer housing in an axial and a rotational directions, and an armature secured to a differential casing in a rotational direction and supported by the differential casing in a manner allowing movement only in the axial direction. A gap is provided between the electromagnet and the armature. The armature is connected to a clutch cage arranged in the differential case. A plurality of friction plates are provided between the clutch cage and a sun gear. The friction plates are alternately connected to the clutch cage and the sun gear. The armature is always pressed by a spring so as to be away from the clutch cage to engage the friction plates one another. When the electromagnet is energized, the armature is attracted to the electromagnet to increase the engaging force between the friction plates, so that a large differential limiting force is produced.
However, in this configuration, when the friction plates are worn away by the differential limiting operation, the gap is changed which is produced between the armature and the electromagnet. A magnetic force is changed in proportion to the square of the gap. Therefore, the change of the gap greatly affects the differential limiting force and there is a fear that a desired differential limiting force is not produced. When the friction plate are further worn away, the gap run out so that the limited slip differential device does not work. The similar device is shown in U.S. Pat. No. 4,860,612.
Another device is shown in U.S. Pat. No. 4,989,686. In this device, a magnetic force from an electromagnet attracts a plurality of friction disc through a rotator to engage the rotator and the friction disc. Consequently, the rotator rotates relatively to a sleeve to move the rotator and the friction disc by a ball cam to engage the clutch plates. However, in this device, the gap between the rotator and the friction disc also changes to greatly affect the differential limiting force, and there is a fear that the clutch plates fall away because of the change of the gap between the friction disc and the clutch plate.